Existing mechanisms to mitigate errors in digital data transmission due to impulse noise can be categorized as either retransmission mechanisms, forward error correction (FEC) mechanisms, or troubleshooting measures.
Retransmission mechanisms consist in repairing corrupted data packets by re-sending the data packets for which a retransmission request has been received. The retransmission request can be issued by the receiver that has received an irrecoverably affected data packet (such implementations are often referred to as ARQ or Automatic Repeat Request mechanisms) or alternatively can be issued by a timer waiting for the receiver to acknowledge the reception of a data packet but not receiving such acknowledgement within a predefined time from the sending of that data packet. Combinations of both, i.e. retransmission requests from the receiver for irrecoverably damaged packets and retransmission requests from a timer in or near the transmitter for unacknowledged or lost data packets, are described in literature as well.
Retransmission is typically done at the higher layers, i.e. the protocol stack layers above the PMD (physical medium dependent) layer such as the TCP layer or Transmission Control Protocol layer.
Retransmission has also been suggested for the physical layer, for instance in the ETSI SDSL standard contribution 054t34 from France Telecom entitled “Impulse Noise Correction in SDSL Using Retransmission Request”. In this standard contribution, it is proposed to implement at the PMD (Physical Medium Dependent) layer of an SDSL (Symmetric Digital Subscriber Line) transmitter recording of recently transmitted data segments. The SDSL receiver can request retransmission of a data segment that was corrupted through the occurrence of impulse noise on the copper pair by indicating the segment number of the data segment to be re-sent. The SDSL transmitter receiving a retransmission request, handles the request in priority.
Other retransmission schemes can be found in A. S. Tanenbaum, “Computer Networks”, Fourth Ed., 2003, more particularly in section 3.3 and 3.4 thereof.
Existing retransmission mechanisms such as TCP retransmission are useless in case of real-time services such as VoIP (Voice over IP) or broadcast TV services because retransmission mechanisms are plagued by high delays due to the inherent latency. Further, retransmission mechanisms are inefficient or useless on communication links with a high frequent, repetitive loss resulting from REIN due to the bandwidth expansion inherent to retransmission.
Forward error correction (FEC) mechanisms are based on the calculation of a FEC code, i.e. an amount of redundant bits or bytes that are added to each data packet and can be used in the receiver's decoder to recover a limited number of transmission errors such as errors due to impulse noise. Popular FEC mechanisms are for instance Reed-Solomon encoding, Parity-Based encoding, Harris Ascent encoding, . . .
If for instance Reed Solomon (RS) encoding is used to cancel impulse noise effects, the digital data words are extended with a redundant Reed Solomon code before transmission. Upon receipt, the redundant Reed Solomon code enables the decoder to detect and correct errors in the digital data words induced by noise. At the price of some overhead, Reed Solomon coding enables to resolve errors resulting from any type of impulse noise. Reed Solomon encoding is usually combined with interleaving techniques in order to spread the errors resulting from a noise impulse over several data words, thereby improving the capability of the Reed Solomon decoder to correct all errors induced by the noise impulse.
Forward error correction mechanisms like Reed Solomon encoding and interleaving are typically configured using a parameter called INP or Impulse Noise Protection. The higher the INP value, the higher the Reed Solomon overhead and consequently the lower the effective data rate on the line. The lower the INP value, the lower the duration of impulse noise bursts against which protection is provided. Optimal configuration of the INP parameter requires monitoring and precise characterization of the impulse noise. In other words, an accurate picture of the impulse noise and how it affects the data packets, e.g. the DMT symbols, needs to be available in order to optimally configure the forward error correction parameters on a digital communication line.
A known method for impulse noise monitoring has been described in the ITU-T agreed baseline text TD 158R1 (WP 1/15) entitled “G.ploam: G.vdsl: Report of ad-hoc session on Impulse Noise Monitoring”, a temporary document, published as a result of an ad-hoc session held on 3 Nov. 2006. In the impulse noise monitoring method described therein and illustrated by the first figure, the remote end terminal senses quality degradation of received DMT symbols and generates an indication for severely degraded DMT symbols. The impulse noise indications are thereafter compacted to basic statistical information like anomaly histograms of the impulse length (IL) and the impulse inter arrival time (IAT). These statistical data that remain available in the known method for INP parameter setting occupy limited storage capacity. However, part of the information on the sensed impulse noise is lost irrecoverably in the prior art impulse noise monitoring method as a result of the statistical data generation. For instance the timing information on individual noise pulses, individual error lengths, individual inter arrival times, the cross-correlation between the error length and the inter arrival time, etc. is lost as a result of which the possibility to precisely characterise the impulse noise for FEC parameter configuration is lost too. Further, as a result of the compacting function being implemented in the remote terminal's software or hardware, it is hardly possible to introduce other histograms or statistics that could improve the INP parameter setting, like for instance two-dimensional impulse length—inter arrival time histograms, Fourier transforms, sliding window based histograms, histograms based on pulse gap bridging, or any interpretation or combination thereof, because this would require an upgrade of the software/hardware of the remote terminals. Such upgrade is a slow process requiring all manufacturers to modify their products, typically as a result of international standard setting or a market-driven demand for a de-facto standard.
Similar impulse noise monitoring methods with same disadvantages are described in the International Patent Applications WO 2005/086405 entitled “Impulse Noise Management” and WO 2006/102225 entitled “Method and Apparatuses of Measuring Impulse Noise Parameters in Multi-Carrier Communication Systems”.
Lastly, in cases where no forward error correction mechanisms are available or where no INP values that are sufficiently high to correct all impulse noise errors, the solution to cope with impulse noise may consist in troubleshooting, i.e. finding the source in the customers home and taking measures in order to remove the impulse noise source. In case of a DSL line for instance, removing part of the inhouse wiring and replacing it by better wires may reduce the impulse noise effects to an acceptable level. To help identify and diagnose the source of impulse noise such that remedial action can be taken to remove the source or alleviate the problem, operators demand solutions for monitoring and characterisation of the impulse noise as close as possible to the physical phenomenon.
Known methods to monitor and characterise impulse noise for troubleshooting purposes are based on Single-Ended Line Testing (SELT). Examples are for instance described in ITU-T standard contributions CD-047 from British Telecom and CD-021 from AT&T. SELT however is an off-line method that involves service interruption. In particular for impulse noise characterisation where observation is needed over relatively long periods, for instance a day or several days, the known SELT based techniques will lead to undesirable or unacceptable long service interruption. In addition, the known SELT techniques are seen as measurement methods implemented centrally. Measuring impulse noise at the central side in order to diagnose impulse noise induced at or near the customer premises, is obviously not efficient. Transferring the known SELT based measurement techniques from the central side to the remote, customer premises equipment raises issues on retrieval of the information to the central side. Furthermore, the known SELT based methods estimate amplitude, duration and inter arrival time distributions, which are inadequate parameters for identification of an impulse noise source.
It is an objective of the present invention to provide a method for monitoring impulse noise which overcomes the above mentioned drawbacks of the prior art methods. More particularly, it is an objective to disclose a method for impulse noise monitoring that enables online, detailed characterisation of impulse noises, and/or identification of the impulse noise sources, without service interruption nor issues to collect information from the remote end terminals. It is further objective to enhance the information available for impulse noise characterisation and impulse noise source identification. Another objective is to simplify the upgradeability of the compacting and processing software/hardware that interprets the information collected through impulse noise monitoring.